Cardiac rhythm systems are used for treating an irregular or unstable heart. The systems include, among other things, pacemakers which deliver timed sequences of low electrical energy to the heart, such as via a lead having one or more electrodes. Leads have been implanted in the body for electrical cardioversion or pacing of the heart. More specifically, electrodes implanted in or about the heart have been used to reverse (i.e., defibrillate or cardiovert) or sense certain life threatening arrhythmias, or to stimulate contraction (pacing) of the heart, where electrical energy is applied to the heart via the electrodes to return the heart to normal rhythm.
Cardiac pacing and/or sensing may be performed by the transvenous method or by electrodes implanted directly onto the epicardium. Traditionally, to attach a lead epicardially, a thoracotomy is performed where the thorax is opened to obtain access to the heart. This procedure involves painful and expensive surgery for the patient. Transvenous pacing may be temporary or permanent. In temporary transvenous pacing an electrode catheter is introduced into a peripheral vein and fluoroscopically positioned against the endocardium.
Traditional permanent transvenous pacing is performed under sterile surgical conditions where an electrode is positioned in the ventricle or atrium through a subclavian vein, and the proximal terminals are attached to a pulse generator which is implanted subcutaneously. The distal tip of the lead is positioned within an apex of the heart to hold the lead in place. Leads which are implanted in the apex of the heart have a backstop, which assists in preventing the lead from floating, and allows for the distal end of the lead to become fixated therein. Potential complications induced by the presence of the lead within the chambers of the heart, however, can preclude lead implantation of the lead within the chambers of the heart.
One approach to resolve this issue is to implant the lead transvenously. When a lead is implanted transvenously, the passage has no termination into which the lead can be positioned, resulting in a floating lead. One of the drawbacks of a floating lead is that the performance of the electrical interface between the electrode and the tissue can be diminished. In addition, the veins or arteries are filled with blood, surrounding the electrode, which further inhibits the electrical interface between the electrode and the tissue.
Defibrillation, which is used to treat a heart which is beating too quickly, delivers a high energy electrical stimulus which allows the heart to reestablish a normal rhythm. In addition to a defibrillating electrode can be combined with a pacing and/or sensing electrode. To obtain lower pacing and sensing thresholds, the pacing/sensing electrode is traditionally disposed at the tip of the lead, which is positioned deep within an apex of the heart at the largest center of mass of the heart. To minimize sensing problems following a shock from the defibrillating electrode, the defibrillating electrode is separated away from the sensing/pacing electrode on the lead. As the defibrillating electrode is moved away from the tip of the lead, however, the shock strength requirement increases resulting in increased demands on the battery of the cardiac rhythm system.
Accordingly, there is a need for an implantable lead that is capable of placement and fixation in other regions of the heart, such as within vascular structures. In addition, there is a need for a lead having an electrode for positioning within a passage, such as a vein or artery, that allows for fixation of the lead. There is also a need for a body implantable lead which allows for effective stimulation from a defibrillation electrode. There is further a need for a lead that is capable of effectively defibrillating the heart, and also pacing/sensing the heart in a limited area.